Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOX storing catalyst which stores NOX contained in exhaust gas when the air-fuel ratio of inflowing exhaust gas is lean and releases stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, NOX generated when fuel is burned under a lean air-fuel ratio is stored in the NOX storing catalyst. On the other hand, if an NOX storage ability of the NOX storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich whereby NOX is released from the NOX storing catalyst and reduced.
In this regard, fuel contains sulfur. Therefore, the exhaust gas contains SOX. This SOX is stored together with the NOX in the NOX storing catalyst. In this regard, this SOX is not released from the NOX storing catalyst by just making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOX stored in the NOX storing catalyst gradually increases. As a result, the amount of NOX amount which can be stored gradually ends up falling.
Therefore, known in the art is an internal combustion engine in which SOX is prevented from being sent into the NOX storing catalyst by arranging an SOX trap catalyst upstream of the NOX storing catalyst in the engine exhaust passage (see Patent Literature 1). In this internal combustion engine, the SOX contained in the exhaust gas is trapped by the SOX trap catalyst and therefore the SOX is prevented from flowing into the NOX storing catalyst. As a result, it is possible to prevent a drop in the storage ability of NOX due to the storage of SOX.
In this regard, in this SOX trap catalyst, when the temperature of the SOX trap catalyst rises, if the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is made rich, SOX will end up being released from the SOX trap catalyst. Further, in this SOX trap catalyst, when the concentration of SOX trapped near the surface of the SOX trap catalyst rises, even if the temperature of the SOX trap catalyst is not high, if the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is made rich, SOX will end up being released from the SOX trap catalyst.
Therefore, in the above-mentioned internal combustion engine, the exhaust gas flowing into the SOX trap catalyst is maintained constantly lean so that that SOX is not released from the SOX trap catalyst and, when the NOX storing catalyst should release NOX, a fuel feed valve arranged downstream of the SOX trap catalyst and upstream of the NOX storing catalyst feeds fuel to make the air-fuel ratio of the exhaust gas flowing into the NOX storing catalyst rich.